Batteries exist in many different physical forms, using various combinations of electrodes. With the advent of electric vehicles and other machinery, the demand for high energy density batteries has increased. This demand has impacted upon battery design. For such applications, it is desirable to manufacture batteries that have electrodes in close proximity to one another. However, when such batteries are constructed, there is an increased risk of the battery developing an internal short-circuit due to electrode proximity.
As one example of the aforementioned problem, reference is made to alkaline nickel-cadmium batteries. Traditionally, such batteries used a sintered negative electrode. Today, that type of electrode has been replaced by a plastic bonded electrode to increase storage capacity. However, when repeatedly charged and discharged, a battery having a such an electrode is more likely to internally short-circuit earlier than such a battery having a sintered negative electrode. This internal short-circuit problem is caused by a phenomenon known as "migration," in which cadmium active material grows and transfers from the negative electrode to the positive electrode.
One method of overcoming the aforementioned problem has been through the use of microporous separators. The growth of cadmium active material is retarded through a microporous separator, and, therefore, this method can substantially completely prevent internal short-circuits caused by the growth of cadmium active material. One problem with such separators, however, is that they have low gas permeation coefficients, creating the potential for undesirable rupturing or venting of the battery. A second problem arises from the inclusion of surfactants in such separators for the purpose of achieving adequate membrane wettability. Such surfactants eventually leach out of the separator, contaminate the battery environment, and cause premature failure and degradation of the battery.
Wettable battery separators are disclosed in U.S. Pat. No. 5,126,219, and these separators are made of filaments and fibers of ultrahigh molecular weight polyolefins that are formed as a web, having a void volume of at least twenty percent. Wettability is imparted by incorporating a finely divided, hygroscopic filler material into the polyolefin extrusion mixture prior to extrusion. Such a formulation imparts a more permanent wettability to the material than is provided by simple coating, but still presents problems in actual practice.
There are numerous battery systems in existence which require the use of a separator. Unfortunately, each system has its own specific requirements with regard to separator properties. However, certain properties of separators are considered to be desirable regardless of the particular battery system in which they are employed:
(a) thin and reliable separation between positive and negative electrodes, PA1 (b) very low electrolytic resistance in the electrolyte, PA1 (c) long term chemical stability upon exposure to the electrolyte and oxidants, even at elevated temperatures, PA1 (d) ability to absorb and retain a large amount of electrolyte, PA1 (e) good gas permeability, and PA1 (f) high mechanical strength in machine and transverse directions. PA1 (a) a porosity ranging from about 50% to about 95%, PA1 (b) an average pore size ranging from about 0.1 to about 20 microns, PA1 (c) an electrolytic resistance ranging from about 1 to about 50 m.OMEGA.-in.sup.2, PA1 (d) a maximum weight loss of 1% and a change in electrolytic resistance of no greater than 25% after immersion in an aqueous solution of 35% KOH and 5% KMnO.sub.4 at 50.degree. C. for 1 hour, PA1 (e) a tensile strength of from about 3.5 to about 5.5 lb/in in both length and width directions, PA1 (f) a KOH absorption ratio of from about 5 to about 30, and PA1 (g) a Gurley air permeability of about 1 to 300 sec/10 ml.
No commercially available separators meet all of these criteria.
Thus, there remains a need for such a battery separator, which concurrently possesses a variety of desirable properties. It is an object of the present invention to provide such a battery separator, as well as a battery incorporating such a separator.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.